Generally, most DC electric tools have speed control functions, and their operating currents are relatively larger. Control devices for DC electric tools are usually power devices like insulated gate bipolar transistor (IGBT), metal-oxide-semiconductor field-effect transistor (MOSFET), etc. Speed control by a larger current will certainly result in rapid heating of the power device, and furthermore the fast rising operating temperature will result in rapid overheating failure of the power device. Currently in order to prevent a power device from overheating, a usual means for measuring the real time temperature is adopted to monitor the temperature by mounting a temperature measurement element on the outside of the power device.
Because heat transferring between objects is a slow procedure, this factor results in that the temperature sampled from practical applications always lags behind the actual operating temperature of the power device. When examining the temperature control of the power devices, the following situations are typically observed:
1) The significant heater of a power device is typically the inner core thereof, and therefore a time lag phenomenon exists during heat transferring from the inner core to the periphery of the power device.
2) Typically, only when the temperature of the whole body of a temperature measurement element reaches the temperature to be measured, can its featured resistance be truly consistent with the temperature to be measured. But a time lag phenomenon exists before the temperature of the whole body of the temperature measurement element reaches to a certain temperature.
3) If a packaged temperature measurement element is mounted on the packaged body or heatsink of a power device, there is a time lag phenomenon of the temperature measurement because the thermal conductivity of the package material of the power device or the temperature measurement element is lower.
4) If the package shape of a temperature measurement element does not closely correspond to the shape of a power device, there exists an air gap therebetween after installation, and because the thermal conductivity of air is lower, a significant time lag phenomenon of the temperature measurement exists. It is oftentimes necessary to fill the thermally conductive material in the gap to improve the thermal conductivity between the temperature measurement element and the power device, but a time lag phenomenon still exists. Moreover, the operation for filling the thermally conductive material into the gap has some uncertainties such that the thermal conductivities of the temperature measurement element and the power device are not consistent with each other in mass production, thus increasing the difficulty of the temperature compensation.
5) According to the internal structure of a power device, the temperature of its heatsink is closer to that of the inner core. When a temperature measurement element is mounted to the heatsink of the power device, as the heatsink of the power device usually has an electric network, the temperature measurement element has to be electrically isolated from the heatsink of the power device, which lowers the thermal conductivity between the temperature measurement element and the power device. As a result, the measured temperature lags the actual temperature of the power device.
Due to the above observed phenomena, the external temperature measurement element can not accurately track the core temperature change of the power device in real time and the real-time monitoring is insufficient for some situations, which may result in the inability to shut down the power device in time and may result in the overheating failure of the power device.